1. Field of the Invention
The present invention relates to a wet-type multi-plate clutch used in a clutch and/or a brake of an automatic transmission (AT) of a vehicle and a method for manufacturing a plate used in such a wet-type multi-plate clutch.
2. Description of the Related Art
Conventional wet-type multi-plate clutches used in automatic transmissions generally comprise separator plates as external toothed plates and friction plates as internal toothed plates constructed by sticking friction materials onto core plates, and are designed to transmit torque by engaging the external toothed plates with the internal toothed plates.
In the past, as shown in FIG. 1 of Japanese Patent Application Laid-open No. 11-141570 (1999), a wet-type multi-plate clutch constituted by sticking friction materials onto both surfaces of friction plates as internal toothed plates is known. Further, a clutch design of one-surface alternate sticking type in which friction materials are stuck onto surfaces of the internal toothed plates and the external toothed plated which are not oppose to each other is also known.
However, in recent years, in order to reduce fuel consumption of a vehicle, it has been requested that the vehicle be made more compact and lighter, and an automatic transmission be made more compact and lighter to reserve the interior space of the vehicle, and also, an axial dimension of a wet-type multi-plate clutch used in the automatic transmission be made smaller.
In the conventional clutches, it is contemplated that a plate thickness of the core plate is decreased in order to reduce an axial dimension of a clutch pack; however, if the plate thickness of the core plate is decreased, heat resistance is reduced due to reduction of a heat absorbing ability of the metal surface of the external toothed plate associated with the friction material, and, since the plate thickness is small, a face pressure of the engagement surfaces between the internal toothed plate and the hub is increased and a face pressure of the engagement surfaces between the external toothed plate and the clutch drum is increased, with the result that inconvenience such as indentation is generated at the engagement surfaces. Thus, the reduction of the plate thickness causes a problem.
In the conventional plates, due to thermal insulation of the friction materials, most of friction heat generated upon engagement of the clutch was dispersed through the external toothed plates to which the friction materials are not stuck.
In the conventional clutches, it is contemplated that, in order to enhance the heat resistance, a plate thickness of the external toothed plate is increased to increase a heat dispersing amount; however, if the plate thickness is increased, there arises a problem that an axial length of the clutch is increased. To cope with this, a wet-type multi-plate clutch of one-surface alternate sticking type in which friction materials are stuck alternately onto one surface of the external toothed plate and one surface of the internal toothed plate so that friction heat is dispersed from both of the external toothed plate and the internal toothed plate has been proposed.
However, in such clutches of one-surface alternate sticking type, since the friction heat is generated at only one surface of each plate, unbalance of heat is generated between both surfaces of the plate, which may cause deformation of the plate, with the result that a bad influence is affected upon endurance and/or a friction property of the friction material. Further, during an idle rotation, since axial load balances between the front and rear surfaces differ from each other, a clearance between the internal toothed plate and the external toothed plate cannot be maintained to a fixed value, thereby increasing idle rotation drug.